In semiconductor manufacturing processes such as, for example, the sputtering of thin films onto substrates such as silicone or gallium arsenate wafers, it is usually necessary to transfer wafers among different process chambers. In some systems, wafers are transferred in a horizontal plane where the wafer typically rests on a transfer arm resembling a paddle. The gravitational force on the wafer and the resulting friction between the wafer and the underlying supporting element, combined with controlled acceleration of the wafer on the support element, makes possible the use of clamp-free wafer transfer. In some systems, however, for process related and other reasons, wafers are moved in non-horizontal orientations or held in non-horizontal orientations during processing. In such systems, the wafer is locked to and carried by a device called a wafer holder that is equipped with clamping structure or other locking elements to hold the wafer in position on the holder.
The locking device of a wafer holder is typically referred to as a latch. The latch exerts pressure on the wafer to lock the wafer in a fixed position on the holder. With certain spring biased latches, for example, the latches exert pressure against a wafer, which is supported by the multitude of independent tabs or by a wafer mounting ring. Friction force between such elements and the wafer prevents the wafer from slipping during the motion of the wafer holder.
Some wafers of which semiconductors are made are very fragile, either because of their small thicknesses or because of the nature of the substrate material of which they are made, for example, gallium arsenate wafers, which are particularly fragile. In the final stages of device fabrication, a large number of nearly completed semiconductor devices on a wafer adds substantial value to the basic wafer, however, making such wafers very expensive. Any breakage of a wafer at such later stages of fabrication represents a substantial economic loss and contributes a substantial cost to semiconductor device manufacturing. Poor function or reliability of the latches that retain the wafers to the holders can result in such breakage as well as increase the equipment down-time and a loss of equipment throughput. Further, rough operation of the latches can cause the production of particulates in the vacuum chamber causing contamination of the wafer, which can interfere with the fabrication of semiconductor devices and result in the production of defective devices.
High vacuum systems are widely used in technical and commercial applications such as semiconductor device manufacturing processes, which require the performance of the particular process in the absence of air and airborne contaminants. An example of such a process is the sputter deposition of thin films onto silicon wafers, or substrates, in the manufacture of integrated circuit chips. With the ongoing trend toward device miniaturization in semiconductor manufacture where device features are currently assuming submicron dimensions and with integrated circuits increasing in functional size, complexity and value, particles of a size and quantity heretofore tolerable in semiconductor manufacture are now capable of causing damage that can totally destroy expensive devices and, therefore, are no longer tolerable.
Still further, in the process of manufacturing semiconductor devices on silicon wafers, there are applications in which a need often exists for sputtering onto the back side of the wafer. For example, gold is sometimes applied on the back of wafers to facilitate heat removal from the chip. Where front side sputtering is taking place, the back side of the wafer is typically sealed against a heating surface built into the wafer holder. During backside sputtering, however, the front side of the wafer, which contains the devices being manufactured, must not touch the backplane.
Wafer holder latches act in concert with the physical elements that define the mounting plane of the wafer holder on which the wafer rests to clamp the wafer between the latches and those elements on the wafer holder. Such latches may include, for example, flexible tabs or a suitable wafer mounting ring. The holders include an appropriate wafer exposure opening to permit etching or sputtering of the wafer. In the prior art wafer holders, the interaction of the latches and the mounting plane structure has produced a number of the problems discussed above.
The holder of the wafer clamping device typically consists of a multitude of flexing tabs and a plurality of latches, for example, three latches, mounted to the wafer holder housing. The flexible tabs define a wafer mounting plane on which the wafer rests. When the wafer is loaded onto the wafer holder, a load arm of a robot typically positions the silicon wafer against the tabs in a precise and repeatable manner so as to minimize chipping and provide reliable wafer latching. The wafer is held in place on the holder with friction generated, for example, by spring loaded rollers forcing the wafer against two adjacent tabs. The force applied by such rollers deflects the wafer locally a small amount in accordance with the thickness of the wafer. Thinner and more flexible wafers tend to induce less friction against the tabs and have a greater tendency to slide during acceleration. If such sliding happens to position a tab at or near the edge of a wafer, wafer chipping can result, particularly when the wafer is sealed against a backplane.
The latch itself can cause problems. The major components of a typical latch assembly are the body of the latch with a front roller and one or more rear rollers mounted opposite the front roller or to its sides. The term "front" roller refers herein to a roller that rolls against a wafer to latch the wafer to a holder, and the terms "rear" roller and "back" roller refer herein to rollers that remain in contact with the holder to guide the latch or to position the latch by registration with detents on the holder. The latch assembly customarily pivots around a rigidly fixed post anchored to the wafer holder, and the assembly is usually held together by a helical spring. The body of the latch typically contains a slot which is engaged by actuator pins on a loading arm. The actuator pins are operated to rotate the latch through an arc of usually 90.degree. to latch and unlatch the latches, moving the front roller into position against or retracted from the wafer. In addition to applying force for holding the wafer in place by urging the front roller against the wafer, the helical spring maintains the latch assembly in an upright position normal to the holder so that unobstructed engagement of the actuator pins with slots in the latch can occur.
When the latches are rotated, they slide against stationary wear shims which are biased by the force of the latch spring. The latched and unlatched positions of a typical latch are defined by three detents 90.degree. apart. Three equal depth detents are typically provided on the surface of the holder into which the rollers of the latch assembly are pushed by the force of the spring, and a fourth detent, which is deeper, is provided closest to the edge of the wafer. The deeper detent allows the wafer engaging portion of the front roller to lower onto the wafer and apply positive pressure of the wafer against the tabs when the latches are in their latched positions. The body of the latch is typically made out of metal, usually stainless steel. The rollers are usually provided with a substantially smaller cross-sectional radius than the spherical radius of the detents in order to minimize traction resistance of the rollers. All mutually moving elements are also typically made out of metal such as stainless steel and employ sliding pairs with various degrees of frictional resistance.
In use, latch actuation is preceded by the loading of a wafer onto the wafer holder such that it rests against the tabs. Simultaneously, the actuator pins on the transfer arm engage the slots of each of the latches, with the rollers expected to be in full detent and the latches in their open or unlatched positions. While the wafer is held with the suction of a vacuum chuck or by some other clamping mechanism on the transfer arm, the latch actuator pins rotate the latches about their pivots. Both rollers of the latch rise out of and descend into a shallow pair of detents, thus pulling the latches into and keeping them in the desired orientations. However, when the front roller is latched onto the wafer, this function is performed only by the rear roller. Generous clearance between the pins and the slots is typical in order to avoid brushing of pins against the slots during the engagement, which is a frequent cause of wafer chipping. Due to such oversized slots, the latch rotation is always less than 100% that of the actuator pins. The force of the spring is expected to overcome frictional resistance of the sliding elements and to pull the rollers fully into the detents, thus completing 100% rotation.
Every time the edge of a misaligned slot is struck by an actuator pin of the transfer arm, the impact is transmitted to the front roller, which can cause a piece of wafer to be chipped away by the roller bearing against the wafer. This results in significant waste of devices and downtime for cleaning. In high vacuum, the use of a slot wider than the pins is not always effective. In many situations, high friction within the latch prevents the latch slot from being kept in alignment with the center of the actuator pins. However, misaligned slots may be due to something as simple as the tilt of the latch caused by a thin wafer. It is, however, more often found that incomplete positioning of the rollers in the detents is the cause of misaligned slots.
Many sliding joints in the design of the latch result in high frictional hysteresis within the latch mechanism, which makes the slot position and its subsequent engagement unpredictable and unstable. Friction among many mutually interacting surfaces are the factors influencing completeness and repeatability of the seating of the rollers in the detents, which is needed to bring the latches fully to their latched and unlatched positions. For example, residual friction between the rollers and the body of the latch can lead to a tilt of the latch. Friction from the rollers, combined with torsional friction between the body and the spring, contributes torsional resistance, which the latch spring is unable to overcome so as to be able to push the rollers fully into the detents. Consequently, less than complete rotation of the latch occurs providing insufficient clamping, particularly of very thin wafers, allowing them to slide on the holder and providing misalignment of the actuator pins with their slots, thereby allowing them to strike the latch bodies instead of entering the slots. If stronger springs are used to overcome such incomplete latch rotation, more wear is generated and the front roller can be caused to snap onto the wafer, which can shatter thinner wafers.
Furthermore, the friction coefficients of materials increase many times in vacuum, and the wear of most materials at temperatures of above 600.degree. C. significantly increases, causing non-metals to abrade and metals to gall, causing highly undesirable particle generation, which causes defective fabrication of semiconductor devices or possibly misalignment of latches and resulting wafer clipping.
Rollers of the prior art also present problems. Small cross-sectional radii of usually identical front and rear rollers contribute to wear, especially at elevated temperatures, which alter the trajectory of the front roller. If the cross-sectional radius of a roller is not identical to that of the spherical radius of the detents, contact stresses increase when rollers roll into and out of the detents. Metallic rollers provided on metallic shafts in ultra-high vacuum of, for example, more than 10.sup.-8 Torr, are subject to phenomenon known as "cold welding", which causes metal-to-metal pairs to stick together. Where front rollers do not roll but, rather, slide, they gouge the rolling surface of the base and also rub the wafer and either score or shift the wafer out of alignment.
Prior art uses of so called soft ceramics, for example, MACOR, produce low friction but exhibit very poor wear properties and do not possess required structural integrity where contact stresses are high, thus rendering them unsuitable as highly moveable parts in particle conscious applications. Rollers made out of these materials are easily dented from the slightest snap into a detent, and, as a result, they must be frequently replaced due to the excessive wear of the shafts and roller surfaces and the formation of dents and flats on the gripping surfaces. Excessive wear of the front roller surfaces and insides of detents alters the trajectory of the front roller causing the front roller to roll over the edge of the wafer rather than to descend gently onto the wafer. Rolling over the edge of the wafer is the major cause for the wafer shift.
The material of which the front roller is formed and that comes into contact with the wafer has been of paramount importance in the art from a process point of view. While hard ceramics, in general, are available with only minute traces of sodium, the soft machinable ceramics such as MACOR contain, in addition to high content of sodium, potassium and fluorine which are totally unacceptable elements for some critical processes of semiconductor devices.
It has also been found that the cross-sectional shape of the wafer-contacting portion of the front roller has direct impact on wafer chipping and particle generation. With rounded edge rollers, the tangent point of wafer contact can move close to the edge of the wafer where it is more likely to cause the wafer to chip. On the other hand, a cylindrical roller can cause the roller to roll onto an edge, which can also generate particles by crumbling the edges of the wafer or the roller.
The phenomenon of cold welding also occurs when the concentrated force of the metal spring is applied against a thin metal shim, and against the metal latch in high vacuum, resulting in a very high friction coefficient. Higher spring force must be used to overcome this friction force, causing excessive wear of the joint, generation of particles and failure to maintain accurate detent because of high residual latching torque. Further, resulting torsion on the spring causes the inner sides of the coils to rub against the post, increasing wear and leading to potential spring failure.
Front metal rollers, which are part of a metal latch used on tab wafer holders, are exposed to direct deposition in a coating process. Some deposited materials (TiN, W, TiW) tend to flake off and jam the rollers, requiring frequent acid cleaning to keep particulate count down. In systems which use tab wafer holders, the backside gas easily floods the gaps around the wafer and the latch cavities, which can frequently ignite secondary plasma during etching or RF-bias sputtering. This situation not only generates large amount of particulates, but it is also a source for re-sputtering of unwanted material onto the wafers.
In the case of backside sputtering, that is, where it is necessary to sputter a film onto the side of the wafer opposite the device side, which leaves the semiconductor devices facing into the backplane, the gap between the wafer and the backplane must be small enough to provide good thermal coupling between the wafer and the backplane yet, at the same time, be large enough to avoid semiconductor devices from coming into contact with the backplane. With the wafer latched against a tab ring rigidly mounted to the wafer holder housing, both the wafer holder and the backplane are sealed against same reference surface, which does not provide for the "growth" of the backplane as a result of thermal expansion. Depending on the temperature of the backplane, the wafer gap may vary significantly and impact the process results or completely close, causing damage to the devices on the wafer.
For the reasons stated above and for other reasons, there remains a need for a better method and apparatus for holding wafers for processing and for latching and releasing wafers from wafer holders.